In vehicles, a transmission is interposed between an internal combustion engine and a driving wheel. This transmission is operated to change the driving force of the driving wheel and traveling speed in order to fit the traveling conditions of the vehicle, which are readily and extensively changed from time to time, so that the internal combustion engine can perform to full extent. As one type of such transmission, there is a continuous speed variable transmission which is designed such that a belt ratio is changed by transmitting power in such a manner as to increase or decrease the radius of rotation of a belt looped around pulleys, each pulley having a stationary pulley element fixed to a rotational shaft and a movable pulley element separably attached to the stationary pulley element. The rotational radius of the belt is changed by increasing or decreasing the width of a groove formed between the two pulley elements using hydraulic oil, for example. Such continuous speed variable transmission is disclosed in, for example, Japanese Patent Early Laid-open Publication No. Sho 64-44349.
A hydraulic control mechanism of the continuous speed variable transmission is provided with a single plate type hydraulic clutch adapted to feed and cut power from the engine in response to pressure exerted by hydraulic oil. This single plate type hydraulic clutch is controlled in various control modes in accordance with signals such as engine speed, opening degree of the throttle gate (valve) of a carburetor, etc.
The control modes of clutch pressure in this hydraulic clutch include, for example, the following patterns:
(1) NEUTRAL MODE
In this case the shift position is N or P and the hydraulic clutch plates have been completely disconnected, the clutch pressure is in the lowest level (zero) and the hydraulic clutch is off or disengaged.
(2) HOLD MODE
In this case the shift position is D or R and the throttle has been separated to indicate no intention to travel, or in case it is desired to reduce speed during traveling to cut off engine torque, the clutch pressure is at such a low level as the clutch plates are just connected and the clutch is in its half clutch state (creep state) under clutch pressure of 3.4 to 4.0 kg/cm.sup.2.
(3) NORMAL START MODE
Engine torque is transmitted to the vehicle wheels under clutch pressure of 5 to 15 kg/cm.sup.2.
(4) SPECIAL START MODE
Engine torque is transmitted to the vehicle wheels under clutch pressure of 5 to 15 kg/cm.sup.2.
(5) DRIVE MODE
In this case the vehicle has shifted to a full traveling state and the clutch plates have been fully connected (clutch lock-up state), or after it has shifted from a start mode, the clutch is almost in a locked-up state, wherein the clutch pressure is at a sufficiently high level so as to be able to fully bear engine torque.
In the continuous speed variable transmission, a control unit is provided in order to control the activation of various valves of the hydraulic control mechanism and the hydraulic clutch.
This control unit inputs various control factors, actuates various devices and equipment, and controls the line pressure to a target value of line pressure by means of closed loop control for feeding back a hydraulic signal from a pressure sensor for detecting hydraulic oil pressure of the hydraulic control mechanism, and by means of open loop control in which feed-back by the pressure sensor is not performed.
That is, the closed loop control is carried out such that a target line pressure is established as a function of opening degree of the throttle gate value, belt ratio, engine speed, etc., a difference between such established target line pressure and an actual line pressure is integrated to calculate an integrated value, the target line pressure is corrected by such obtained integrated value, and feed-back control is performed so that the actual line pressure becomes the target line pressure. Also, open loop control is carried out such that feed-back control is not performed and the actual line pressure becomes the target line pressure.
For example, at a start mode time such as a normal start mode (NST) and a special start mode (SST) among control modes of clutch pressure, a filtered line pressure target value (PLINSPF) of line pressure control is established such that the belt and the hydraulic clutch do not slip even with the maximum torque which the internal combustion engine generates at the current opening degree (THR) of each throttle gate. Such established value is calculated with reference to a schedule or map (a throttle/pressure converting map, and a map between the line pressure target value (PLINSP) and the opening degree (THR) of the throttle gate) which is initially input into a program. That is, as is shown in FIG. 5, the filtered opening degree (THRF) of the throttle gate is input, and the line pressure target value (PLINSP) is calculated with a throttle/pressure converting map (401). A filtered line pressure target value (PLINSPF) is found by multiplying a low-pass filter (402), and a duty value or duty rate is found with a pressure/solenoid duty converting map (403). Then a value of correction as an integrated value of the closed control is added to this duty value (404), the corrected duty value is subjected to a limiter (which defines upper and lower limit values) (405), and finally a line pressure solenoid duty (OPWLIN) is output.
In one such control method, in order to absorb aging change and adjustment displacement of the hydraulic circuit, the amount of integration of the closed loop control is used as a value of correction for the open loop control (Japanese Patent Application No. Sho 63-302728).
At start mode time, clutch pressure was heretofore controlled by deciding a target clutch pressure (CPSP) by means of feed forward control and speed loop control. At that time, when the clutch friction coefficient was lowered due to aging change of the hydraulic clutch and oil temperature increase, the target clutch pressure (CPSP), as shown in FIG. 6, was considerably higher (shown by CPSP 1 of FIG. 6) than a regular clutch pressure (shown by CPSP 2 of FIG. 6).
However, in the conventional line pressure control, since clutch pressure is not taken into consideration at start mode time, sufficiency of line pressure with respect to clutch pressure becomes less and, for that reason, there is an inconvenience in that a smooth clutch pressure control becomes impossible.
Also, since output of the internal combustion engine is lowered (i.e. the load increases) when such auxiliary devices as an air conditioner and power steering are used or when the vehicle is traveling on a high ground (elevation), the target pressure (CPSP) is considerably lower (shown by CPSP 3 of FIG. 6) than the regular clutch pressure (CPSP 2). In this way, even when the target clutch pressure (CPSP) is low, it is controlled to the ordinary line pressure target value and therefore, the clutch pressure is increased more than necessary, thus resulting in an increased load on the internal combustion engine.
Therefore the invention provides, in an attempt to obviate the above-mentioned inconveniences, a continuous speed variable transmission control method in which clutch pressure can be normally controlled against changes due to clutch aging change and oil temperature change, a proper line pressure control can be attained, and load on an internal combustion engine can be reduced by controlling the line pressure in such a manner as to reflect the clutch pressure control state of the hydraulic clutch when open loop control is being effected in the start mode.
In order to achieve the above, the present invention provides a continuous speed variable transmission which uses hydraulic oil for increasing or decreasing the width of a groove between a stationary pulley element and a movable pulley element separably attached to said stationary pulley element to respectively decrease or increase the radius of rotation of a belt looped around said two pulleys, thereby to vary the belt ratio. A continuous speed variable transmission control method is characterized in that a hydraulic clutch is provided, connection and disconnection of said clutch being controlled by various kinds of control modes, and a control portion is also provided, said control portion controlling line pressure both by means of closed loop control for performing feedback in accordance with a hydraulic signal from a pressure sensor and open loop control which does not perform the feed back, the line pressure being controlled by said control portion in such a manner as to reflect the clutch pressure controlling state of said hydraulic clutch when said open loop control is performed in the start mode.
According to this method of the present invention, since line pressure is controlled in such a manner as to reflect the clutch pressure control state of the hydraulic clutch when open loop control is performed in the start mode, line pressure is controlled in such a manner as to reflect the clutch pressure control state even when the clutch friction coefficient is lowered due to clutch aging change and oil temperature change, and a smooth line pressure control is ensured. In addition, the load on the internal combustion engine can be reduced by preventing the line pressure from becoming higher than necessary even when the output of the internal combustion engine is lowered.